The relationship between tree height and leaf area: sapwood area ratio

Abstract

The leaf area to sapwood area ratio (A :A) of trees has been hypothesized to decrease as trees become older and taller. Theory suggests that A :A must decrease to maintain leaf-specific hydraulic sufficiency as path length, gravity, and tortuosity constrain whole-plant hydraulic conductance. We tested the hypothesis that A :A declines with tree height. Whole-tree A :A was measured on 15 individuals of Douglas-fir (Pseudotsuga menziesii var. menziesii) ranging in height from 13 to 62 m (aged 20-450 years). A :A declined substantially as height increased (P=0.02). Our test of the hypothesis that A :A declines with tree height was extended using a combination of original and published data on nine species across a range of maximum heights and climates. Meta-analysis of 13 whole-tree studies revealed a consistent and significant reduction in A :A with increasing height (P<0.05). However, two species (Picea abies and Abies balsamea) exhibited an increase in A :A with height, although the reason for this is not clear. The slope of the relationship between A :A and tree height (ΔA :A/Δh) was unrelated to mean annual precipitation. Maximum potential height was positively correlated with ΔA :A/Δh. The decrease in A :A with increasing tree size that we observed in the majority of species may be a homeostatic mechanism that partially compensates for decreased hydraulic conductance as trees grow in height.

Relationships Between Tree Height and Carbon Isotope Discrimination

“…In contrast, homeostatic shifts in plant structure, which operate to hold p c constant within species and sites (Ehleringer 1993;McDowell et al 2006), should push the D versus h relationship towards a linear pattern. Such shifts are well documented to occur with increasing h, such as changes in leaf area:sapwood area and root area:sapwood area ratios (Magnani et al 2000;McDowell et al 2002b;Mencuccini 2003) and sapwood permeability (England and Attiwill 2007). We propose that D moves away from the expected 1/h pattern at low heights due to h-driven homeostatic shifts (McDowell et al 2002a, b;Mencuccini 2003) and light-driven leaf-photosynthetic shifts (Fig.…”

“…McDowell et al 2002b). The theoretical regression should be non-linear according to the simplest version of Darcy's law (D = 1/h), however, we saw no evidence of deviations from linear patterns.…”

Ecosystem CO2/H2O fluxes are explained by hydraulically limited gas exchange during tree mortality from spruce bark beetles

“…The g c response to VPD (the second most significant driver) did not change during the epidemic ( Table 2). As spruce age, they are among a few plants whose stomatal response to VPD is less tightly regulated to plant hydraulics [Ewers et al, 2005], allowing for low leaf water potentials and leading to morphological changes of higher leaf area to sapwood area ratio [Ewers et al, 2005;McDowell et al, 2002]. If stomatal response to VPD was critical to prevent hydraulic failure in this ecosystem, then as the composition of the canopy transitioned during the epidemic from old spruce to young and small spruce and fir, the g c response to VPD should have become more sensitive, which is the case for trees with higher reference conductance ( Figure 3a).…”

Within-crown variation in leaf conductance of Norway spruce: effects of irradiance, vapour pressure deficit, leaf water status and plant hydraulic constraints

“…McDowell et al [18] suggested that the path length from bulk soil to leaf rather than tree height per se is the relevant term. As trees grow taller, G T declines causing stomata to close earlier in the day to restrain water losses and prevent the development of damaging water potential gradients.…”